Keepin' It Real (Estate): Banks Value Your Mortgage, Not Your Home

How Wall Street got primed for subprime loans.

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As the debate rages about whether or not we're finally approaching a floor in home prices, let's examine the value of another asset: The mortgage.

When considering a home-buying transaction, buyers (and sellers) typically worry most about the value of the house. Lenders, on the other hand, are much more concerned with the value of the mortgage.

From a lender's perspective, the economic value of a loan is its expected future cash flow in the form of interest payments. The key word in that phrase -- and why a loan's value isn't purely derived from its rate -- is "expected."

To a bank, a loan is just a product, like an iPod is to Apple or a BlackBerry is to Research in Motion. The value of that product is just how much someone will pay for it. Loans with higher coupons, adjusting for risk, are worth more than those with lower coupons, because they fetch more on the open market.

Mortgages, or debt of any kind, are priced relative to their face value, or "par." A $100,000 mortgage that's worth par would cost $100,000. Prices are then expressed as a percentage of par. That is, a loan with a face value of $100,000 that's worth 102.50 (percent) would cost $102,500.

Subprime loans are often considered "bad," while prime loans are presumed to be "good." Many assume, therefore, that high-quality prime loans are worth more than those lousy subprime ones. This isn't entirely accurate.

"Good" loans are the ones where you're appropriately paid for your risk, whereas "bad" ones are those in which you're on taking too much risk relative to return. As Professor Sedacca often says, "If you aren't being paid to take risk, don't take risk."

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